Normally, there is no counter-rinsing pipe or no effective counter-rinsing pipe in existing riverbed permeable ditches. They can only be built in highly permeable riverbeds of considerable thickness. These permeable ditches catch impurities in water by using natural or artificial permeable layers as filters to attain the object of purification of river water. During the functioning of these permeable ditches, the amount of impurities caught by the natural or artificial filters may continuously increase due to the changes in water quality, resulting in that these permeable ditches commonly produce turbid permeated ditch water (usually with turbidity higher than degree 5), and resulting in the blockage of the permeable ditches. This, in turn, causes a decrease in the water produced per unit time with an increase of the functioning time of the permeable ditch, thereby, the requirements of the design can not be attained.
The riverbed purification filtering apparatus described in Chinese Patent CN86104417 is an example of using artificial filters. In the disclosed apparatus, a purification filtering layer is positioned in the space sandwiched between the inner and outer well walls and confined by the tapered bottom of the well and a cover of the well wall, wherein rinsing porous branch pipes are provided adjacent to the outer well wall. In this device, remaining impurities in the filtering layer can be removed to keep the quality of water of the water source always clear, as well as to prevent the draw down phenomenon of the well. However, this apparatus still belongs to the field of side permeating filtering which can only be built in permeable riverbeds of certain permeability. Furthermore, the well wall cover is a reinforced concrete plate, which makes construction and maintenance of the filtering layer complex.